Seismic exploration is widely used to locate geologic formations for hydrocarbon accumulations. Subsurface formation analysis has enabled more efficient oil and gas recovery over the past several decades. In recent years, petroleum exploration has been occurring at increasingly deeper levels of water. As the water levels increase and the wells drilled lengthen, the subsurface formations often become more complex. To facilitate more efficient petroleum recovery, it is often desirable to generate a vertical seismic profile.
A vertical seismic profile (VSP) is a class of borehole seismic measurements used for correlation between surface seismic receivers and wireline logging data. VSPs can be used to tie surface seismic data to well data, providing a useful tie to measured depths. Typically VSPs yield higher resolution data than surface seismic profiles provide. VSPs enable converting seismic data to zero-phase data as well as enable distinguishing primary reflections from multiples. In addition, a VSP is often used for analysis of portions of a formation ahead of the drill bit.
Narrowly defined, VSP refers to measurements made in a vertical wellbore using acoustic receivers inside the wellbore and a seismic source at the surface near the well. In a more general context as used herein, however, VSPs vary in well configuration, the number and location of sources and acoustic receivers, and how they are deployed. Nevertheless, VSP does connote the deployment of at least some receivers in the wellbore. Most VSPs use a surface seismic source, which is commonly a vibrator on land, or an airgun, marine vibrator, watergun, or other in-sea seismic source in marine environments.
There are various VSP configurations including zero-offset VSP, offset VSP, walkaway VSP, vertical incidence VSP, salt-proximity VSP, multi-offset VSP, and drill-noise or seismic-while-drilling VSP. Checkshot surveys are similar to VSP in that acoustic receivers are placed in the borehole and a surface source is used to generate an acoustic signal. However, a VSP is a more detailed than a checkshot survey. The VSP receivers are typically more closely spaced than those in a checkshot survey; checkshot surveys may include measurement intervals hundreds of meters apart. Further, a VSP uses the reflected energy contained in the recorded trace at each receiver position as well as the first direct path from source to receiver while the checkshot survey uses only the direct path travel time.
While VSPs can provide valuable information about a formation, source perturbations (e.g., shot to shot variations in the seismic signature of an airgun) introduce error into the raw seismic data which percolates through the processing chain to the final images produced. VSP source perturbations can limit the full range of usefulness that VSP data can provide.
Further, some seismic systems include a cluster or array of airguns. Some of these systems fire all of the airguns simultaneously. The intent of the multiple airguns is to increase the amplitude of the seismic signal. However, the vertical spacing between the airguns and sea-surface results in an offset of the signals and what is sometimes referred to as “ghosting.”
In consequence of the factors discussed above, and others that are known in the art, data acquired in marine borehole seismic often does not record with sufficient signal fidelity to be useful, i.e., the frequency content of the source signature is compromised. Typically, currently available techniques correct only gross errors in VSP source data. Therefore, in some instances sophisticated seismic data processing methods may not be used because current borehole seismic methods and apparatus do not consistently provide VSP seismic source data with the precision necessary to make sophisticated processing meaningful.